The invention pertains to primary carpet backings comprising at least two different layers of fibers.
Nonwoven primary carpet backings for tufted carpets have been known for many years. The nonwoven primary carpet backings used in carpet production are either nonwovens comprising a backbone of polyester fibers, e.g. PET, or nonwovens comprising a backbone polypropylene fibers. The term fibers as used herein refers both to staple fibers and filaments.
In general the polypropylene or polyester fibers are filaments in the range of 1 to 25 dtex, preferably in the range of 2 to 20 dtex, most preferably in the range of 5 to 15 dtex providing the required processing stability. The unit dtex defines the fineness of the filaments as their weight in grams per 10000 meter.
The backbone fibers can be entangled by mechanical needling and/or hydroentanglement with fine water jets and optionally bonded with a chemical binder.
Alternatively, the backbone fibers can be thermally bonded, for example by calendaring or through air bonding or the like, using a lower melting polymer which is present in the form of fibers or which has been added as a powder or as granulate. When the lower melting polymer is present in the form of a fiber, the polymer can be present as separate monocomponent fibers or the lower melting polymer can be present together with the backbone polymer in the same fibers in so-called bicomponent or multicomponent fibers. Bicomponent fibers are well known in the art, for example as side-by-side, core/sheath or segmented pie fibers. Alternatively, fibers composed of a single polymer but with varying crystallinity along the length of the fiber can be used as bicomponent fibers as varying crystallinity results in fiber sections consisting of polymer with different melting points along the length of the fiber. Such an effect can for example be obtained by stretching the fibers to different degrees in a spin-draw process to obtain varying crystallinity and thus varying melting point along the length of the fibers as disclosed in WO 93/13254.
In case both the backbone polymer, such as for example polyester or polypropylene, and the lower melting polymer are present as separate monofilaments so-called bifilament nonwoven types are created. Alternatively, the lower melting polymer can be present in the same filament as the backbone polymer thus forming bicomponent filament nonwovens.
WO 03/033801 discloses multilayer nonwoven carpet backings wherein the composition varies in different sections in a single nonwoven layer to influence the mechanical properties of each section within this single layer in order to facilitate optimised moulding of tufted car carpets.
U.S. Pat. No. 6,808,786 discloses a two-part primary carpet backing for improved acoustic properties. The first layer is a woven or nonwoven material. The second backing layer is formed of microfilament spunlaced material to obtain sound absorbing acoustic properties. Both layers are bonded together in a separate production step with an adhesive web.
The process of making a tufted carpet comprises the steps of providing a primary carpet backing material into a tufting machine, tufting the backing material with tufting yarns to obtain a greige carpet with a face side showing the tufts and a back side where the back stitches are formed. A tuft is the visible part of the tufting yarn on the face side of a greige or carpet. A back stitch is the visible part of the tuft yarn on the backside of a greige carpet.
Over the years, nonwoven primary carpet backings have been improved constantly in order to meet the ever more stringent demands of carpet manufacturers and consumers. New tufting techniques such as high-low tufting to produce carpets with high tufts and low tufts in a single carpet require primary carpet backings with improved stitch holding capabilities to hold the tufts in place in the tufting process. Of particular importance are improving the appearance of the finished carpet, allowing effective mending of imperfections which occur during tufting, and facilitating tuftability, i.e. by providing lower tuft-needle penetration resistance and reduced needle deflection during tufting, avoiding backtags, increasing stitch holding performance and reducing noise during tufting.
The newly formed tufts in the greige carpet are held into place by the stitch holding capacity of the primary carpet backing material. Stitch holding is the force required to remove a loop from a greige carpet from the back side. It determines how strong the interaction is between the primary carpet backing and the tufting yarn. The stitch holding determines the chance of failures in the subsequent processing steps after tufting until a pre-coat has been applied to lock the tufts in place. These processing steps can comprise dyeing, pre-coating, tentering, steaming and/or transport of rolls of greige carpet. Each of these processing steps are executed under severe conditions regarding temperature and/or applied tensions, which pose high demands to the primary carpet backing material. The tuft-bind performance of the pre-coat determines mainly how well the tufts are held in place in the pre-coated carpet. After pre-coating a heavy layer can be coated onto the back side of the pre-coated carpet and/or a secondary carpet backing can be applied.
The primary carpet backing should exhibit good tuft holding characteristics. The primary carpet backing should have a high stitch holding capacity to hold the tufts in place during the dynamic process of tufting to obtain tufts of essentially the same height for an even carpet surface in the greige carpet. The stitch holding capacity of a primary carpet backing is the ability to put the tufts in place during the tufting process where the tufts are formed and to hold the tufts in place, directly after being formed, during the formation of the following tufts. It determines the face and backtags of the greige carpet during the tufting process. A backtag is a loose or uneven backstitch in a greige carpet, in essence a tuft which was formerly on the carpet face that is now (partially) on the back side. During subsequent processing steps, like dyeing and precoating, the stitch holding of the primary carpet backing should be high enough to retain the carpet surface as produced during tufting.
To lock the tufts in the greige carpet into place a pre-coating can be applied to the back side of the greige carpet. The tuft bind is the force required to remove one tuft completely out of the (pre-coated or finished) carpet from the face side. It determines how strong the adhesive bond is between the pile fibers and the adhesive coating of the primary carpet backing. The pre-coat is generally applied either as a latex solution or as a foamed latex to the back side of the greige carpet and subsequently the latex is dried to form bonds between the base of the tufts and the primary carpet backing.
If the pre-coat has a too high affinity to the primary carpet backing material the pre-coat can migrate completely through the primary carpet backing to become visible at the face side of the carpet. This is known as latex bleeding, which is undesirable in certain carpet constructions.
If there is little adhesion of the pre-coat to the primary carpet backing material the pre-coat will not form sufficiently strong bonds between the base of the tufts and the primary carpet backing to keep the tufts into place during further processing. Furthermore, problems arise at cutting of the selvedges when the primary carpet backing is not well bonded to the other components of the finished carpet construction.
Normally the tufts cover the entire surface of the carpet, but under certain conditions the primary carpet backing can become visible during the use of the carpet, especially when the carpet is installed on non-flat surfaces. An example of this is the use of such nonwovens in automotive carpets. On the basis of the geometry of the car body floor the carpet has to be bent, i.e. deformed, strongly in various areas. Especially pronounced of course is the deformation in the area of the transmission tunnel. In such cases it is beneficial for the primary carpet backing to have the same color as the tuft yarns. When pre-dyed tufting yarns are being used to produce a carpet the primary carpet backing should have essentially the same color as the pre-dyed tufting yarns. However, it is not economical for the primary carpet backing manufacturer to produce different nonwoven primary carpet backings for each available colour of tufting yarn and at the same time it is not economical for the carpet manufacturer to have large amounts of different coloured primary carpet backings on stock.
Alternatively the primary carpet backing can be tufted with non-dyed, so-called raw white, tufting yarns. The greige carpet can be dyed in a subsequent processing step. It is preferred that the primary carpet backing has good dye-ability to avoid a (too large) colour mismatch between the dyed tufting yarns and the dyed primary carpet backing which would render the primary carpet backing still visible when being bent. Moreover, carpet constructions exist wherein the primary carpet backing is visible even without bending the carpet. In principal it is only required to dye the top side of the primary carpet backing which is oriented to the face side of the carpet.